The invention relates to a method for connecting overlapping flat parts according to the generic term of claim 1. The invention also relates to a device for connecting overlapping flat parts according to the generic term of claim 4.
From DE 40 09 813 C1 a device for connecting overlapping flat parts is known. Using this device sheets can be joined by the so-called clinching process. What is so special about this device is that in the perimeter range of the die recess a deformable material is provided which is softer than that of the sheets to be joined. This is to enable avoiding the usual division of the die.
From DE 39 23 182 C2 a device with a divided die is known for connecting flat parts arranged over each other using a punch. To support the components of the die a body is provided with complementary conical surfaces and on this body a return spring for preloading in the direction of the punch movement.
In clinching processes, using devices according to DE 40 09 813 C1 or DE 39 23 182 C2, a punch penetrates the sheets to be joined in a linear movement during the working stroke, while a fixed or divided die on the opposite side takes part in forming the so-called die side of the clinching spot. A counterpunch is positioned in the divided die.
The forming of the recess by the tools not only produces the effect of force closure but also the known form or self-closure, the so-called undercut, between the parts to be joined with the process carried out up to a pre-determined bottom thickness.
It is a disadvantage of clinching that great forces are required. This results in high loads acting on the tooling and the tooling frames, and limits the application of the method when high-strength sheet materials have to be joined.
Great requirements of the guidance of the tools result from the heavy joining loads, if high-accuracy coaxial alignment is required.
Heavy loads reduce the possible daylight of the C-frames, which are mainly used as tooling frames, and hence, limit the applicability of the method.
Because of the linear punch movement the thickness of the punch-side sheet is significantly reduced in the neck range and only little undercut is established, which limits the strength of the joint.
Superimposition of an orbiting motion onto a joining feeding movement is known from the method of orbital riveting. Orbital riveting is used for achieving self-closed connections by partial upsetting and stamping operations, e.g. at an auxiliary joining component.
It is the objective of this invention to reduce the forces to be applied for clinching. This aims at extending the field of application of this process concerning high-strength materials and the accessibility of C-frames for large workpieces. It is another objective of this invention to eliminate the process""s weak points of low neck thickness and small undercut and thus to achieve higher joining strengths in equivalent joining jobs. Further, it is intended to make complicated dies unnecessary and reduce the demand for coaxial positioning of the punch and die as far as possible.
According to the invention, the problem is solved by a method with the features mentioned in claim 1. The subclaims present other useful developments of the invention. Further, the problem is solved by a device with the features mentioned in claim 4. Useful developments of the invention follow from the features mentioned in claims 5 to 9.
In this method the axial feeding movement is superimposed with an orbital motion of the forming punch and a varying punch force with this orbital motion and varying punch force tuned to each other.
While the punch performs an orbital motion the punch force is controlled such that the punch force is increased during the travel of the contact area produced between the punch front face and the workpiece outwards from the centre of the clinching spot, and is decreased during the inward travel of this contact area.
Owing to the orbital motion the material is partially deformed so that the process forces are distinctly reduced. The varying punch force leads to the desired radial material flow from the centre in outward direction.
The said orbital motion can be superimposed to the axial feeding movement during the whole joining process or during a portion of the joining process.
According to the invention the forming punch is equipped with a mechanically driven mechanism, which sets this punch in motion. Further, a mechanism powered mechanically, servohydraulically, piezoelectrically or in other ways, is provided which feeds the forming punch with a varying force against the recess. The application of the varying punch force to the forming punch is evaluated and controlled dependent on the orbital position.
On the opposite side, the sheets are supported by a fixed or moving die with or without counterpunch, or on an even anvil only.
A special geometry of the forming punch in connection with the orbital motion ensures that more material is drawn into the forming zone at the beginning of the joining process. During the deformation and above all, at the end of the process material is radially pressed from the bottom area into the neck area, which is critical for the strength of the connection.
For this to occur the forming punch has a front end with radii R1/R2 and/or a taper and/or a tractrix curve, whereby when shaped with radii only, the radius R1 is greater than the maximum punch diameter and the radius R2 is smaller than the maximum punch diameter.
The counterpunch has a front end with a radius R3 and/or a taper and/or a tractrix curve, whereby when shaped with a radius only, the radius R3 is greater than the maximum punch diameter.
Advantageously, the forming punch has a punch undercut where the maximum punch diameter is tapered in such a way that any collision between the shank of the forming punch and the cylindrical portion of the punch-side sheet is avoided.
It is advantageous that a movable die has a die undercut where the die interior diameter is tapered to support the undercut at the joining spot.
Also a fixed die can be used. This fixed die should have a draft with an angle and/or transition curves between the die head and bottom.